1. Field of the Invention
This invention relates particularly to the integrated circuit industry but more generally to any industry where an application is needed to have a uniform thin film of material formed or deposited on the surface of an object.
There is increasingly a need in various industries and in particular the integrated circuit industry, to be able to provide thin films of controlled thicknesses or coatings of material on surfaces of substrates of various sizes and complexities. The size of these surfaces range from those of small surface areas up to relatively large flat panel displays and large area mask requirements. Present coating systems and methods do not satisfy all of the technical needs and requirements for these applications.
2. Description of the Prior Art
Various methods are presently known and used for coating the surfaces of objects and in particular, the coating of substrates as required in the integrated circuit industry.
Spin coating, where the coating material is deposited on a substrate surface and then the surface is rotated, is known to provide good film uniformity for sizes of substrates less than 10" in diameter. This method, however, results in inefficient coating material usage and a limitation on the size of the surface or substrate that can be coated, due to the requirement for rotation of the object.
Dip coating, where the object to be coated is immersed in a liquid source of the coating material, may be more efficient but it typically results in a film which is not very uniform in its thickness or easily reproducible. Although large size objects can be handled, there is a need for a large liquid reservoir. The dip coating method also has the disadvantage that it does not provide for a closed environment and therefore the coating material and system is susceptible to contamination resulting in defective layers being formed on the surfaces of an object.
Forming a film of material by spraying the material on a surface results in the difficulty to control thickness uniformity for films which are often needed to be less than 1 to 2 microns in thickness. This may result in the efficient use of a coating material and relatively large sized surfaces can be handled, but once again it is not a closed system.
Meniscus coating methods are known whereby a coating material is flowed through a porous applicator onto the surface of the object to be coated. Meniscus of the coating material is formed between the porous applicator and the surface of the object to be coated and the meniscus is maintained during relative movement between the surface and the applicator. An example of such a procedure is described in U.S. Pat. No. 5,270,079 which issued Dec. 14, 1993 to Hendrik F. Bok, entitled "Methods of Meniscus Coating" and assigned to Specialty Coating Systems, Inc. This described procedure results in efficient use of the material. However, the application is limited as to the size of the object that can be coated due to its reliance on the proximity of the physical applicator to the surface of the object in order to maintain the meniscus. The larger the size of surface to be coated, the more difficult to maintain the positioning of the physical applicator with respect to the surface, and therefore variability in the meniscus and hence the film thickness results. Again, this is not a closed environment and contamination of the coating material could result.
Other representative prior art references and the associated shortcomings of their teachings including further details of some of the above methods are now presented.
U.S. Pat. No. 4,590,094 entitled "Inverted Apply Using Bubble Dispense", issued May 20, 1986 to Frederick C. Ringer, Jr., and is assigned to International Business Machines Corporation. The description is of a method and apparatus for coating a substrate comprising a spindle holding a workpiece in an inverted manner so that the face of the workpiece faces downwardly towards a liquid deposit of the coating material which is supported in a nozzle beneath the spindle. The spindle is moved to permit the inverted workpiece to contact the liquid deposit and the spinning spreads the coating material uniformly across the surface of the spinning workpiece. This is an open system and therefore the coating material is susceptible to contamination and would be used in a clean room environment for best results. The size of the substrate that could be coated would be limited to what could be accommodated by the apparatus and the final curing of the coating would be done in a separate bake oven.
U.S. Pat. No. 5,188,669 entitled "Circuit Board Coating Apparatus with Inverting Pallet Shuttle" issued Feb. 23, 1993 to William E. Donges et al. The coating apparatus described has one coating station to coat both opposite surfaces of two-sided circuit boards. One upwardly facing surface of a first circuit board is coated at the coating station and moved to a handling station, while a second circuit board is moved to the coating station where its upwardly facing surface is coated and the first circuit board is inverted in the handling station. The procedure is then continued such that both sides of the circuit boards are coated. A single overhead spray coating station results in both sides of the circuit board being coated without the coating station being idle when the boards are being inverted. Thus, alternate sides of a circuit board are coated and if curing of the coating is required, this would be done in a separate bake oven.
U.S. Pat. No. 5,223,037 entitled "Plant for the Manufacture of Printed-Circuit Boards or Multi-Layers", issued to Niko Kraiss et al on Jun. 29, 1993. This reference describes adjacent modules providing continuous and interconnected processes for producing electrically conductive coatings on printed circuit boards. After a circuit board is prepared at previous modules, the circuit board is passed through a coating module while submersed in the coating liquid before being moved to the final module where the organic coating is polymerized.
U.S. Pat. No. 5,234,499 entitled "Spin Coating Apparatus" issued to Tadashi Sasaki et al on Aug. 10, 1993 and describes spin coating apparatus for use in applying a coating solution to form a film on an upper surface of a substrate. The apparatus includes a rotary table for holding and spinning a substrate in a horizontal manner. An upper rotary plate is parallel to and spaced from the upper surface of the substrate which defines a flat treating space between them. A nozzle plate is disposed between the rotary table and the substrate support which dispenses fluid which is directed to the surface of the substrate.
U.S. Pat. No. 5,415,899 entitled "Method of Forming an Organic Thin Film" issued May 16, 1995 to Toshio Nakayama et al. The described method includes creating organic molecules having a hydrophilic portion and a hydrophobic portion on a water surface. The substrate to be coated is pulled across the surface thereby transferring the portions of the monomolecular layer onto the substrate.